The greening of potatoes is a quality problem that causes substantial losses to growers every year. This phenomenon has been the subject of numerous studies and publications, and is better explained in the following references.
In a first publication entitled: "The Potato, Third Edition" by W. G. Burton, pp. 274, 311 and 517, (1989), the author explains that if potatoes are exposed to light they develop chlorophyll, particularly in the peripheral layers thereof. This can happen when, for example, the tubers protrude out of the row, or are insufficiently covered by soil. The author also explains that the greening reduces the acceptability of the product on the market, and because of the parallel development of solanidine, greened potatoes are sometimes bitter when cooked. Moreover, the reference also teaches that the effect is not merely cosmetic or flavour, greened potatoes can be unfit for consumption because of a high content of glycoalkaloids.
In a second reference entitled: "The Potato Crop, The Scientific Basis For Improvement", edited by P. M. Harris, pp. 319-320, 360, 507-508, 572-573, (1978), the editor explains that the major cause of greening is insufficient cover over the tubers when the tubers form near the soil surface. The reference also suggests that the incidence of greening is caused mainly by tuber variety, stem density, the size of seeds planted and ridge size and construction.
The ridging of potato rows is normally effected after planting, when the foliage is well deployed and the tubers start to develop. This operation is normally carried out with conventional hilling implements that have not changed greatly over the years.
In this regard, a multiplicity of hoeing and hilling implements have been developed in the past for hilling potatoes. A first example of a conventional machine for hoeing potatoes is illustrated in Canadian Patent No. 154,781, issued in 1914 to C. A. Hayden. The hoeing portion of the machine comprises a pair of moldboards that are set diagonally to one another and are adapted to be drawn over a ridge of potatoes, one on either side of the ridge. In use, the moldboards scrape the soil between the ridges and move this soil over the side and top portions of the ridge between the moldboards.
In another example, the U.S. Pat. No. 1,103,770, issued on Jul. 14, 1914 to P. E. Hogan illustrates and describes a single moldboard implement. This device has a shovel-like tooth that is adapted to loosen up the soil between the ridges. When the implement is pulled between rows of cultivated plants, the shovel-like tooth loosens up the soil and the moldboard moves the loose soil laterally against the row of plants.
It may be appreciated that although these implements have undeniable merits, they are lacking the ability to compact the soil against the ridge, and therefore a newly formed ridge is susceptible to erosion.
In a third example, the U.S. Pat. No. 2,096,766, issued on Oct. 26, 1937 to G. H. Self, describes a potato hiller that has blades on the forward side of each moldboard for cutting weeds. This potato hiller also has a pair of packing plates hinged to the rear side of the moldboards for packing the soil on the sides of a row of potatoes for example. Each packing plate is urged downward by a pivoted rod and a spring.
Although the inventor of this latter potato hiller has recognized the benefit of packing soil against a ridge for preventing erosion and for protecting potatoes against sunburns and frost, this potato hiller has not enjoyed a lasting success. It is believed that the particular mounting of the packing plates is mechanically complex and is subject to wear and frequent maintenance. Further, it is believed that because each packing plate is set at or below the cutting edge of a moldboard, and makes a reflex angle with the cutting edge and with the forward side of the moldboard, the packing plates cause the implement to float over the soil, thereby reducing the ability of the moldboards to efficiently scrape and move loose soil over the ridges.
As such, it may be appreciated that there continues to be a need for a new and improved hilling spade that is capable of simultaneously scraping soil and compacting the loosened soil against a ridge of potatoes for forming a lasting cover over the potatoes in that ridge.